And tonight, we're investigating a grisly murder committed in broad daylight at a college community. Just as shocking the location, the victim, a professor and family man. And now, his friends and former students remember this beloved teacher. Who killed him? And what was the motive? ABC's Matt Gutman has the mysterious clues. Okay. What's going on with him? I don't know. The driver's side window is all bashed in. And he's got blood all over his head. Reporter: A 911 call, punctuating the morning calls. A vivid description of Florida state law professor, Dan Markle, in his car, dying. I don't know if someone tried to shoot him or he shot himself. I don't know. Reporter: This is isn't the domain of gunshot victims, but college professors and family men, like Markle. Who dropped his sons off at day care. In his garage, left for dead, until his neighbor found him, checking on the curiously open garage door. I heard about break-ins in the area. And then, I saw the window shattered. Reporter: But Markle wasn't robbed. The police said the assailant waited for him and killed him in his garage. He's still moving around. He's alive. Reporter: Markle would die 14 hours later at Tallahassee memorial hospital, just a few minutes' drive from the house. The sandy-haired scholar with an ivy league pedigree, leaving behind his sons, 2 and 5. And a deepening mystery, rattling this tight-knit community. I cannot imagine a person wanting to hurt him. I don't think any of his friends can. He's unforgettable. Very unique personality. Reporter: The big questions, what happened to the conscientious professor and father of two? And who would want to kill him? We do believe that this was not a random act, where he just surprised somebody. What we do believe is that Mr. Markle was the intended victim. Reporter: In other words, murder. The investigation begins with in a 911 call. 12, long minutes describing the gruesome scene, from a 72-year-old neighbor. And I came back over. And his driver's side window was shattered. And he's spattered and can't answer. Reporter: And that first clue. Another neighbor telling police about a silver prius speeding away. No one saw the person come or go. Maybe they saw the car. Is this professional? Is this a crime of passion? What do you think? It can be a professional or someone that just wanted him dead that isn't a professional. We can't tell. Anybody could have gotten that access and committed this offense without being noticed by anyone. Reporter: But it was noticed by someone, Markle himself. Multiple sources tell ABC news, Markle was on the phone when he arrived home, remarking to the person on the phone that, there was someone in his driveway. He did not sound threatened. We do believe this was not a random act, where he just surprised somebody. We believe that Mr. Markle was the intended victim. It's imperative for police to question the ear witness as much as possible. That person listened to this deceased professor moments before he perished. So, every detail could become relevant. Reporter: Minutes later, that silver prius seen speeding from the scene. A vehicle of interest. But what about a person? Harvard and cambridge-educated, Markle had a controversial blog. Could this be clue number two? He posted a blog about the death penalty. The same website on which he received threatening comments in 2012. One commenter saying that, Markle needed to be stopped by all means necessary. Is it possible that some of his scholarship and literature created some enemies? That's as possible as any of the other scenarios. Reporter: And there are several. Markle had been married to another eminent lawyer, Wendy Jo Adelson. They had those two boys. But their love soured. Markle described the day she left him as his pearl harbor. Adelson said through her lawyer, she is just devastated and scared to death. Police tell ABC news, they questioned his ex-wife. Sources say it lasted for hours. But police say they questioned many who knew him. And the fact there are no suspects, at least officially, that doesn't mean the police aren't looking at certain people? I hope they have their eye on certain people. And they're just not telling us. Reporter: Only one witness could positively I.D. The killer, Markle, bleeding out in his car. You hear the neighbor in that 911 call, urging E.M.S. To hurry. And told they were on their way. They better be if this guy's got a shot, living. Reporter: 19 krushle minutes passed. The dispatcher blew it. He screwed up. Had they gotten there earlier, he may have survived. Reporter: On that 911 call, you can hear the dispatcher's confusion. I'm changing the -- Reporter: The man who runs Tallahassee's dispatch center, tells ABC news the operator didn't hear the caller mention the gunshot wound at first. It was entered as a man down, versus a gunshot wound. Reporter: All of that creating a toxic brew of confusion and delay in the response. As one E.M.S. Unit was replaced by another. 19 minutes is a long time. We're making changes in this review to make sure this atmosphere doesn't happen again. Reporter: His blog, once used to share his opinions. Now, a place for those who knew him to share their grief. Late this week, investigators increased the reward for any leads to $10,000. Hoping precious clues might help unravel this mystery. For "Nightline," I'm Matt Gutman, in Miami. Thanks to Matt Gutman for that report. And stay with ABC news for the latest details on this

This transcript has been automatically generated and may not be 100% accurate.

911 Call Released in Florida Professor's Death Investigation

Confusion during the emergency call led to the EMT taking 20 minutes to arrive at the scene.

New Details Arise in the Murder of FSU Professor

Evidence shows that an alleged killer waited for Dan Markel to arrive home before shooting him in his own garage.

Police Baffled by FSU Professor’s Fatal Shooting

Authorities are scrambling to unravel the mystery killing of Dan Markel, a prominent law professor.

Shot FSU Professor Waited 19 Minutes for Ambulance

Reward for information about Dan Markel's murder increased to $10,000..

FSU Professor Was Talking on Cell Phone When Shot Rang Out

Dan Markel's killer was lurking outside his home, source says.

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